1. Field
Exemplary embodiments relate to a photorefractive dendrimer compound and applications thereof, and more particularly, to a photorefractive dendrimer compound including a non-linear chromophore containing a tricyanopyrroline-based electron-withdrawing group and a carbazole derivative having excellent charge transport properties, a method of preparing the photorefractive dendrimer compound, a photorefractive device including the photorefractive dendrimer compound, and a method of manufacturing the photorefractive device.
2. Description of the Related Art
A photorefractive material is a material having a refractive index that varies depending on the intensity of light that is applied thereto. Generally, a photochemical reaction is used to change the refractive index depending on the intensity of light. Such a photochemical reaction is irreversible, and is thus suitable for use as an information storage medium, but is unsuitable for use as an information processing medium.
Therefore, there is a need for a material the refractive index of which may be reversibly controlled depending on the intensity of light. To this end, not a photochemical reaction but a photophysical phenomenon should be used. The photorefractive material having photocurrent and electro-optcal phenomena is a typical material able to reversibly control the refractive index. When the photorefractve material is exposed to light, electric charges are generated therein in proportion to the intensity of light due to the photocurrent thereof. The electric charges thus generated are moved by diffusion or drift. As such, the difference in the concentration of the electric charges within the material occurs to thus form an internal space-charge field. Further, because this material may represent an electro-optical phenomenon, the refractive index of the material changes in proportion to the magnitude of the internal space-charge field. That is, the refractive index of the material is reversibly changed in proportion to the intensity of light, which is referred to as a “photorefractive phenomenon”.
The photorefractive phenomenon was first discovered using LiNbO3 in 1966 by Ashkin, who considered the phenomenon to be nothing more than damage to the material caused by a laser. However, after one year, Chen in the Bell Institute revealed that the phenomenon is caused not by laser damage but by a fanning phenomenon based on the reversible refractive index change. Accordingly, the photorefractive phenomenon came to be recognized as a new optical phenomenon. Although photorefractive material using an inorganic material has been proven to be applicable as various optical materials, it has not yet been commercialized because the above material is difficult to process and expensive inorganic material is used. On the other hand, the photorefractive phenomenon of an organic material was first discovered from organic crystalline COANP (2-cyclooctylamino-5-nitropyridine) including TCNQ (7,7,8,8-tetracyclo-quinodimethane) in 1990, and was observed using a crosslinked epoxy-based non-linear polymer material containing DHE (diethylamino-benzaldehyde diphenylhydrazone) as a charge transport molecule the following year. Organic material is receiving attention because it is relatively easy to prepare, is inexpensive, thus enabling mass production, and may have easily controllable chemical components and structures, thus making it suitable for various applications. However, the organic photorefractve material still has many problems, including a low response speed, high electric field for application, and poor durability, upon application to real-time information processing.
Recent research into photorefraction is focused on organic photorefracton, which is reaction in response to near infrared light, and is used in biological applications. The biotissue has high light transmittance at wavelengths of 700˜900 nm. When near infrared light is radiated onto the tissue, not only light reflected from the surface of the issue but also various scattering rays are simultaneously emitted, depending on the depth of penetration of light into the tissue. In this case, when the scattering rays are selectively sorted, depending on the depth, and are then imaged, an image of the tissue section may be realized from two-dimensional images taken at different depths. This is the bio-imaging technique.
W. E. Moerner reported a remarkable gain coefficient of 370 cm−1 under near infrared light (830 nm) using a mixture including two types of organic glass (Appl. Phys. Lett., Vol 82, pp. 3602 (2003)). However, the device manufactured using the low-molecular-weight material mixture is known to have poor durability due to crystallization, and furthermore, has a very low response speed, undesirably decreasing the applicability thereof as a device for near infrared transmission holograms. Therefore, although it is important to have a high gain coefficient, a material having superior device stability and a high response speed is required for actual application to hologram devices.